Smith G.T. Cutting Tool Technology: Industrial Handbook

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Figure 220. A typical UHSM spindle cardridge listing some of factors aecting such a spindle’s design and its operation.

Machining and Monitoring Strategies 443

Figure 221. Attainable cutting parameters – with diering milling spindles, plus HSM is aected by the feedrate and distance

to be traversed, prior to the desired velocity being achieved – for conventional slideway motions

444 Chapter 9

NB Pneumatic spindles can be rotated at excep-

tional speeds, by virtue of the ‘ideal condition’ of

minimal metal-to-metal contact, although one

serious disadvantage being they suer from a low

power output, or to be more specic – torque.

•

Hybrid spindles (Fig. 214-top) – have been de-

veloped to answer the major drawback to utilising

pneumatic spindles. e hybrid spindle as its name

suggests, is a combination of conventional ball-

bearing and pneumatic spindles. Here, the spindle

design incorporates an aerodynamic thrust bearing

with transversal spiral grooves (Fig. 214-top right)

thereby creating an intense pressure wave prole,

which can withstand up to 300% greater static loads

to that of a conventional aerostatic bearing. A typi-

cal hybrid aerodynamic spindle bearing allows the

assembly to achieve rotational speeds ranging from:

0,000 to 40,000 rev min

–1

, with >15.5 kW power at

peak speed.

NB H

ybrid spindle cartridges are signicantly less

expensive than the magnetic ‘active’ spindles, but

more expensive than pneumatic spindle cartridges.

In both of these latter versions, they have a rela-

tively long in-service life, as wear-rates are mini-

mised, but do not have the stock-removal capability

of the former cartridge.

In Figs. 220 and 221a, are shown some of the principal

factors that aect UHSM spindle performance. In Fig.

220, these factors are represented in an Ishikawa (i.e.

‘Cause-and-eect’) diagram. Here, the many of the in-

ter-related eects can be seen, although other factors

can also be added, depending upon the local condi-

tions of usage: cutting data; workpiece material; wet,

dry, or near-dry cutting; together with the machine

tool’s overall condition.

9.2 HSM Dynamics –

Acceleration

and Deceleration

If the HSM spindle cartridges – mentioned above –

are tted to conventional CNC machine tools, or the

more likely low-cost scenario would be to simply t a

echanically-driven speed-increaser. en the result

of this HSM spindle tment, will enable high rotational

speeds to be produced, but it leaves the CNC-proces-

sors somewhat compromised in its ability to produce

the desired acceleration and deceleration capabilities.

As a practical example of the problems likely to be

encountered, the graphs produced in Fig. 221b and c,

were drawn from an industrial HSM machining expe-

rience at a precision metrology company’s premises,

using several of the latest vertical machining centres

with the spindle of each machine, being tted with

a mechanically-driven speed-increaser (i.e. see Fig.

243a).

By utilising the inboard CNC clock – having a reso-

lution of 0.0001 seconds, the elapsed times for slide-

way motion over varying distances was established. In

Fig. 221b, an exponential relationship is depicted for

the X-axis, this being a typical situation for the other

axes on the machine tool. By a determination of the

required motional distance to attain specic veloci-

ties, it was possible to illustrate the restrictive nature of

both acceleration and deceleration for small slideway

motions. In Fig. 221c, this illustrates the eect of the

required distances to be executed at various velocities.

From Fig. 221c – by way of an example, if a feedrate of

,000 mm min

–1

was utilised, then it would be neces-

sary for a minimum movement of the slideway to be

6 mm to momentarily achieve the desired feedrate,

which is typical for a machining centre having an ac-

eleration of 1.08 m sec

. is physical problem in ac-

tual positioning to the required component’s dimen-

sional feature is not too great a problem for long linear

feeding distances – as the slideway velocity could be

reached, but acceleration and deceleration becomes

exacerbated by the smaller more intricate prismatic

features normally found on the more minute, or

smaller parts oen produced by HSM milling opera-

tions, leading to potential scrappage problems.

From the results of inspection procedures con-

ducted on the HSM over a range of standardised

testpieces, it was concluded that virtually all of the

detrimental dimensional eects introduced by the

HSM milling operations, could be attributed to severe

‘servo-droop’ – more will be mentioned on this subject

shortly. Prior to manufacturing the HSM testpieces,

they were designed on a CAD system and their respec-

tive tool paths were post-processed by the integrated

CAM soware. Hence, with regard to machining cycle

times, they were either calculated by the CAD/CAM,

or were the actual in-cut times – see Table 15. With re-

Machining and Monitoring Strategies 445

gard to these cycle-times for testpiece manufacture, a

signicant improvement accrues when utilising HSM

milling techniques. Although the increase of actual cy-

cle-time from that of the theoretical high-speed CAD/

CAM estimation, can be due to a number of factors

as previously noted by Smith and Hanson (1993). Not

least of which was found in this case, where the CAM

system tended to under-estimate the actual time to

machine a component feature. is time-dierence is

marginally compounded by the ‘servo-droop’ eects.

If the machine tool’s G61 (‘Exact-stop’) mode was

employed, the actual cutting time in general showed

only a marginal increase, over the normal HSM cut-

ting time, although the dynamics of motion tended to

be somewhat jerky in action as the command ensured

it reached its targeted positions. At normal HSM mill-

ing performance, another exacerbating reason for the

increase in cutting time over theoretical, was attribut-

able to the axis acceleration/deceleration parameters

(i.e see Figs. 221b and c). us, these machine tools

basically failed to reach the required slideway accel-

eration/deceleration then maintain these velocities for

about 20% of the total in-cut times, this was for a com-

ponent of somewhat moderate dimensional size and

pocketing intricacy (i.e. the overall testpiece dimen-

sions were approximately: 150 mm in squareness, by

50mm deep).

From the testpiece results, various remarks can

be made concerning the advantages of employing an

HSM strategy over conventional milling practices,

these are:

•

Despite a reduced cut depth, the HSM cycle times

are a 66% improvement over conventional milling

production techniques,

•

Using HSM it will signicantly reduce burr forma-

tion – although not entirely eliminating it, when

compared to that of conventional practice,

•

Distortion of the thin wall features was minimised

by HSM,

•

When employing a speed-increaser (Fig. 243a), its

bearing’s stiness is critically important in order to

obtain an acceptable milled surface texture.

Finally, by utilising even the most elementary form of

HSM approach – using a speed-increaser, highlights

the production advantages to be gained from adopting

this strategy, albeit for limited periods of continuous

cutting time, which normally dictates such ‘increaser’s’

practical usage.

9.2.1 HSM Dynamics – Servo-Lag

Most of today’s CNC machine tools use ‘proportional

servo-systems’ , where the axis velocity is proportional

to the dierence between the actual position and the

command position (Fig. 222a). is ‘error signal’ is

utilised by the system to determine any acceleration/

deceleration necessary as well as the steady-state ve-

locities. As one can visualise from Fig. 222a, the dis-

tance between the actual and commanded positions is

commonly termed ‘servo-lag’. is explanation can be

taken a stage further in Fig. 222b, where the illustra-

tion depicts how a ‘proportional servo-system’ is used

to mill a sloping line. In this example, DX and DY are

the total programmed changes in position on both

the X- and Y-axes, respectively, to go from point ‘A’ to

point ‘B’. Conversely, DX

and DY

are the amount of

lag on each axis at point ‘C’ along the tool’s path from

‘A’ to ‘B’. Furthermore, in such a system the lag on the

X-axis must be proportional to a similar lag in the Y-

axis, in order to accurately follow the slope of the line.

is aect can be mathematically-represented by the

following relationship:

= Slope of the line

In Fig. 222c, we can gain an appreciation of just what

happens when the servo-lag on both axes is not pro-

portional. As the machine tool’s axes travels from point

‘A’ to point ‘B’ , the lag on the X-axis is proportionally

Table 15. A comparison of the theoretical and actual ma-

chining times for the manufacture of testpieces, by various pro-

duction routes

Machining

Method:

Theoretical Cad/

Cam Time:

Actual In-cut

time:

Conventional 3.86 3.71

High-speed 1.26 1.50

High-speed (G61)* 1.16 1.55

NB All times in minutes

* G61 is the ‘Exact-stop’ mode of machine command and, when ac-

tivated, the machine tool will not initialise another movement un-

til the previous axis command has been completed (i.e the target-

point), thus ensuring an accurate and nal slideway positioning.

[Source: Smith and Maxted, 1995]

446 Chapter 9

Figure 222. The CNC control problem of servo-lag and its aect on the associated HSM motional kinematics.

Machining and Monitoring Strategies 447

less than the lag on the Y-axis. is error might be the

result of the ‘servo-gains’

between the X- and Y-axes

not being properly synchronised. Normally, ‘servo-

gain’ can be expressed in units of: mm min

–1

[i.e. velo-

city / mm (i.e. distance in 0.001)] of lag. us, lag can

be determined using the following relationship:

Lag L (mm) =

Feedrate F

Gain G

, 

�.

= . mm

For example:

If a machine tool’s moving axis is travelling along

its slideway at 2,500 mm min

–1

and the servo has a gain

of 2, the lag will be 1.25 mm, as indicated in the fol-

lowing calculation:

L (mm) =

, 

�.

= .mm

9.2.2 Effect of Servo-lag

and Gain on Corner Milling

If two axes with correctly matched servo-lags can

move in a straight line from point ‘A’ to point ‘B’ , then

to comprehend the eect of gain, let us consider what

occurs when milling a right-angled corner at a con-

stant feedrate without stopping (Fig. 222d).

Whilst milling the corner from ‘A’ to ‘B’ and the

onward to ‘D’ , the servo develops a steady lag (DX

until sucient command signals have been generated

to reach point ‘B’. It is at this position that the con-

trol begins to generate commands toward point ‘D’ ,

although the actual slideway has not yet reached point

‘B’ , owing to the servo-lag (DX

). At this point the X-

axis will begin to decelerate and, simultaneously, the

Y-axis begins to accelerate, that is the velocity is pro-

portional to the distance between the command signal

and the actual position. Acceleration factors aect the

slideway motions producing the result that the dis-

tance from ‘B’ to ‘C’ is always greater than DX

. Fur-

thermore, this curved path is not a circular arc, but an

exponential curve, with the amount of variance from

the sharp right-angled corner being dependent on the

12 ‘Gain’ or to be more specic: ‘servo-gain’ , in this instance, is

a measure of the servo’s responsiveness. us, the higher the

gain, the lower the lag.

magnitude of servo-lag

, which itself depends upon

the aect of feedrate and gain – according to the previ-

ous formula.

9.2.3 Effect of Servo-Lag and Gain

Whilst Generating Circular Paths

For one to fully understand just what happens when

milling complex contours, it will be helpful to consider

the simple case of a milled path where two straight

lines are joined by a semi-circle (Fig. 222e). In this

situation, the milling operation occurs at a constant

feedrate moving from point ‘A’ in a straight line until

the command dimension reaches point ‘B’ . However,

at this point, because of the eect of servo-lag, the axis

motion will have only reached point ‘B

’. erefore, as

the control command is moving forward at a constant

rate, it begins to generate commands toward point ‘C’.

is action results in the axis motion beginning to

move away from the desired path at point ‘B

’. e dot-

ted line depicted in Fig. 222e, shows the actual path

taken by the cutter and as one can visually observe,

from points ‘B

’ to ‘C

’ , the deviation from the desired

path is shown as ‘e’.

In this example, the magnitude of ‘e’ is determined

as a function of the: feedrate; gain; plus the desired

radius. When the radius error approaches the pro-

grammed radius, the resulting machined prole ap-

pears distorted and is hence, impracticable. Speci-

cally, if one needed to mill a 25 mm radius at a feedrate

of 2,500 mm min

–1

with a machine tool gain being:

25 mm/min/0.001, then the error generated would

be approximately 0.125 mm, equally, if the gain was

increased to 100 mm/min/0.001, the maximum er

ror ‘e’ will be considerably reduced to approximately

0.008 mm.

A machined curve is an approximation on CNC

machine tools, in that the prole is constructed from

a series of short connected segments, or chords. e

controlling factor on the length of such segments is

the deviation between the centrepoint of any chord

13 ‘Servo-lag’ , is sometimes referred to in the literature as:

‘Servo-droop’ – due to its ‘rounding-eect’ at the corners, this

being particularly prevalent when fast feedrates are selected,

creating fast tool path velocities, particularly when normally

undertaking high-speed milling operations.

448 Chapter 9

and a point at right angles on the programmed curve.

e linear distance between these two points is usually

termed the ‘maximum allowable chordal deviation’ and

is a function of the CNC controller’s executive soware.

So, the resultant machined curve is a combination of

the chordal deviation and the servo-lag for a particu-

lar machine tool. To illustrate this condition, Fig. 220f

shows the culmination of servo-lag when following

a contour, with the curve ‘C1’ being the desired con-

tour, ‘C2’ a linear approximation (i.e. the programmed

path), ‘C3’ is the actual generated path resulting from

servo-lag utilising a high gain servo and nally, ‘C4’

being the path generated by a low gain servo. rough

servo-lag, a smoothing of any contour occurs owing to

the lagged cutter path, this causes severe contour prob-

lems with respect to part accuracy and precision for

the simple arc geometry depicted in Fig. 222e. Clearly

then, servo-lag and gain promote a variety of eects

on complex shapes, depending upon their geometry

and tolerance, with these errors becoming still more

complicated when one considers three-dimensional

milling contouring. In many circumstances the cutting

of three-dimensional proles may necessitate utilising

four, or more axes with either one, or two of them be-

ing rotary axes being necessary to create the required

tool paths to produce the component. e servo-lag

and gain on all axes must be considered when manu-

facturing complex part geometries. Regardless of the

workpiece’s geometry, or the number of axes utilised,

there is one factor that should be emphasised concern-

ing potential errors created by servo-lag. If servo-lag

is extremely large, then this ‘lag’ can easily exceed the

positioning errors in the machine tool’s basic speci-

cations.

9.2.4 CNC Processing Speed

Possibly the main factor limiting contouring speed is

the CNC’s inherent processing speed, with each pro-

grammed-block

generated for every axis having to

14 ‘Programmed blocks’ , these are basically the ‘G-’ and ‘M-’

and ‘Auxiliary-codes’ which make up each individual block’s

line, with successive blocks in a logical sequence containing

the whole CNC program. Generally speaking, the smaller the

number of blocks – for the successful production machining

of the part, the more ecient and rened has been the pro-

gramming. (Smith et al., 1993)

be read, interpreted, the activated to obtain dynamic

slideway motions. is CNC exercise is usually re-

ferred to as the ‘block processing time’. e maximum

allocated time for block processing of information is

dependent on the length of slideway stroke (i.e. chord

length) and its associated feedrate. It is possible to cal-

culate the maximum block processing time (T

), as

follows:

Maximum stroke length

Feedrate

For example: if we require a prole’s chord length (i.e.

stroke length) of 0.50 mm, in order to maintain con-

touring accuracy whist milling at 3,000 mm min

–1

, or

50 mm s

–1

, with a maximum block processing time,

then this ‘time’ should be less than:

.

, �

.



= . s, or  ms

Many CNC’s have block processing times typically

within the range of 30 to 60 ms, as can be seen from

the above example, the CNC program would suer

from ‘data starvation’ , whilst the controller attempts to

catch up on its data processing. Such ‘starvation’ would

cause hesitation in the slide motions, slowing down

the cutting time and leaving ‘dwell marks’

on the ma-

chined workpiece’s surface. Since this ‘data starvation’

eect is unacceptable, a lower feedrate must now be

programmed to overcome the problem and as a re-

sult, the cycle-time increases. In the above example,

if an older CNC was tted to the machine tool with

the controller’s block processing time being 60 ms, the

cut would have taken six times longer to generate the

prole, than a more modern CNC controller having a

processor capable of 10 ms.

So that we can fully-comprehend the CNC process-

ing speed problem, let us now consider two widely dif-

fering machining applications:

1. Complex three-dimensional milling of a hob – to

manufacture a die utilised in the production of in-

tricate and expensive military metal buttons. Such

15 ‘Dwell marks’ , here are the result of an ‘untimed delay’ in the

program’s execution, created in this instance, by data starva-

tion (i.e. block processing speed was simply not fast enough).

ese untimely delays in the activation of programming

blocks cause the rotating cutting tool to rest and press against

machined surface and thus, generate minute ‘gouging-eects’

in the surface. (Source: Seames, 1990; Smith, 1993)

Machining and Monitoring Strategies 449

a hob will more than likely have very ne detailed

work on its surface, perhaps with radii as small as

.25 mm, requiring a tool tip radius of 0.025 mm.

In order to machine the button’s elaborate features

with such a small milling cutter, the spindle speeds

ight need to reach 40,000 rev min

–1

, utilising a

feed per revolution of 0.008 mm, giving a feedrate of

320 mm min

–1

. Many production engineers would

not consider this as an example of high-speed mill-

ing, but let us look more closely at this particular

machining problem. If the controller has a servo-

ain of 4, with a feedrate of 320 mm min

–1

, this

means that the servo-lag would be 0.75 mm min

–1

which is consistent with milling radii of 0.25 mm.

However, if the servo-gain was 1, this would cause

servo-lag of 0.320 mm min

–1

and in this case, it

obviously could not machine that button’s intri-

cate detailing. In such circumstances, it would be

necessary to appreciably reduce the feedrate to

ay, 75 mm min

–1

to generate the button’s contours,

leading to the cycle-time increasing by 400%. Let

us also now consider the impact of block process-

ing time under these conditions. To mill a radius as

mall as 0.25 mm, we would need to produce linear

stroke lengths of just 0.075 mm – to reproduce ac-

ceptable button detailing. is intricate contouring

ork requires a block processing time of 15 ms. If

the CNC controller has a block processing time

f just 60 ms, then the feedrate must be limited to

5 mm run

–1

which again, increases milling time by

a factor of four.

2. E

CM pattern electrode for a Turbine fan (i.e large

aluminium casting) – here, the electrode’s geom-

etry has very gentle three-dimensional curves. In

this situation the chosen CNC machine tool’s mill-

ng spindle has a 250,000 mm min

–1

capability, cou-

pled to adequate power to cut at a feed of 0.25 mm

rev

–1

. is production requirement produces a feed-

rate of 62,500 mm min

–1

(i.e. being the product of:

250,000 x 0.25) would be possible. For accuracy

and precision, a chordal deviation (C

) of 0.005 mm

would indicate a stroke length of 0.75 mm – if the

minimum radius of curvature for the Turbine fan’s

eometry was 25 mm. Assuming that the servo-

gain of 1 was available, then we would obtain errors

s large as 0.125 mm and with such errors, the ma-

chine tool would not produce an acceptable part.

urther, at 62,500 mm min

–1

, if the block process-

ing time (T

) was 60 ms, this ‘timing’ would require

stroke lengths of 2.5 mm instead of the 0.75 mm we

needed for the required accuracy and precision.

erefore, in order to eliminate the eects of low

gain, or slow processing time, it is necessary to de-

press the feedrate, resulting in the cutting time be-

ing increased up to 400%.

When considering these two practical examples from

a metaphorical sense, the former method can be com-

pared to that of racing a go-kart on a small tight track,

while the latter method is similar to a highly tuned

sports car racing on a longer and smoother track. e

o-kart may only reach speed a of 30 km h

–1

, whereas

the sports car may hit speeds of >200 km h

–1

. e

corner forces and reaction times are similar for both

methods, even though the speeds are vastly dierent.

Looked at from yet another viewpoint, we can say that

the frequency of response of both the drive and car,

that is their servo-gain and processing time, are very

imilar in both examples even though the speeds (feed-

rates) are radically dierent.

In the day-to-day production environment, the du-

plication of specic and precise contours is the end re-

sult of a combination of many inter-related factors. As

the number of machine tool axes required to produce

sculptured part surfaces increases, the diculty of ob-

taining the desired prole also becomes proportion-

ally problematic. So, machine tools that would nor-

mally produce excellent general-purpose machining

work, may not be either accurate, nor ecient enough

to manufacture complex part contouring geometries.

at is, unless their CNC processors can achieve block

rocessing speeds of <10 ms, with servo-system gains

of up to 4, having sucient ‘look-ahead’

capabilities

(i.e. see Fig. 222g) that are required, for any realistic

and practical HSM applications.

16 ‘Look-ahead’ facilities, are when the controller has the abil-

ity to look-ahead through the following sequenced program-

ming blocks* to determine successive motions and actions

– an important feature for any HSM applications. Many of

today’s sophisticated CNC controllers can look-ahead through

a considerable number of these blocks, thereby prompting the

controller’s response, prior to undertaking any command exe-

cutions. *A ‘Block’ can be dened as: A set of words, characters,

digits or other elements handled as a unit – hence, the term

‘block’ – which creates a sequence of lines of a computer pro-

gramming language, that can then be activated upon by the

machine tool‘s CNC controller, producing the necessary pro-

grammed-actions. (Source: Smith et al., 1993)

450 Chapter 9

9.3 HSM – with Non-

Orthogonal Machine

Tools and Robots

Variax/Hexapod – Design Concept

Non-orthogonal machine tools such as the one simu-

lated, designed and developed for HSM applications

is typically illustrated in various ways in Figs. 223 to

225: utilising ‘virtual’ six axes kinematics (i.e. namely:

X-, Y-, Z-, A-, B- and C-axes), therefore these axes

operate without having any ‘true’ slideways. is par-

ticular kinematic concept has actuators that cross each

other forming X’s instead of meeting at apexes to form

triangles, as they occur in aircra ight simulators,

which uses conceptually similar mechanisms – known

as Stewart platforms, these congurations being a

form of ‘parallel kinematic link mechanism’ (Fig. 223).

To develop this new concept for a machine tool, the

manufacturer utilised computer-aided technology

which played a pivotal role in creating the structural

design (Fig. 223). In particular, the application of a

totally three-dimensional design environment was

employed, utilising both nite element analysis (FEA)

in conjunction with kinematic analyses. However, the

‘Variax’ design uses a range of uncomplicated, or stan-

dard mechanical components in its design. While new

forms of motion actuators were discarded in favour of

conventional and well-proven ballscrew technology,

with its accompanying motor and drive machinery.

Even the gimbals that secure the legs at the base and

the spindle carrier, are relatively simple devices.

With the design of such a high thrust machine, a

signicant problem to overcome was the connection

of the spindle to the six legs (Fig. 223a). e answer

to the connection problem was a simple space frame

design, allowing all the forces to be either in tension,

or compression along the structural elements – similar

to a bridge design. If one compared this ‘Variax design’

with that of a ‘plate-type design’ to secure the spindle

to the legs, then the former space frame concept im-

proves the mass-to-stiness ratio by 275%. While an-

other key development problem to be overcome was

that of how a spindle supported and driven by six

axes kinematically moves in space, moreover, was it

even mathematically possible to control the motional

members? For example and by way of illustration of

this complex mathematical/control problem: a simple

‘X-axis’ linear kinematic translation requires all six

legs to simultaneously move, but each leg will move at

dierent speeds, either accelerating, or decelerating at

dierent rates through the whole ‘linear movement’ –

requiring very complex multi-axes mathematical solu-

tions to achieve this action. By employing a system of

novel mathematical transformation runs in real-time

by the CNC’s multi-processor this mathematical trans-

lation action was achieved, but from a programmer’s

viewpoint, conventional ‘word-address format’

programming knowledge was all that was needed to

successful operate the machine tool.

Non-Or thogonal Versus Orthogonal

Machine Tool Designs

e ‘Variax’s’ machine capabilities and benets dif-

fer signicantly from those found on conventional

slideway-based orthogonal machine tools. So, on an

orthogonal machine the slideways must be perfectly

straight, parallel and at 90° to each other. On these

machines, an axis must have accuracy and precision

control along the slideway having linear and rotary

degrees of freedom carefully managed by the ground

way being scraped to minimise any impending errors/

uncertainties. As mentioned earlier, these orthogonal

axes have kinematically 21 degrees of freedom, with:

linear motion; rotational – i.e. yaw, pitch and roll; plus

17 ‘Word-address format’ of CNC programming, can be consid-

ered as: A system of coding instructions whereby each word in

a block is addressed by using one, or more alphabetic characters

identifying the meaning of the word. [Source: Valentino and

Goldenberg, et al. 2000]

For example, some typical ‘G- and M-codes’ are:

G00 – rapid movement/ traverse of the tool (modal);

G01 – linear interpolation (ie. tool moved at a prescribed fee-

drate) (modal);

G02 – circular interpolation clockwise – CW (modal);

G03 – circular interpolation counter clockwise – CCW

(modal);

G04 – programmed dwell (*non-modal);

G40 – Cancel cutter diameter compensation (modal);

G41 – tool diameter cutter compensation (i.e. radial-oset) on

le-hand side of workpiece (modal);

G42 – tool diameter cutter compensation (i.e. radial-oset) on

right-hand side of workpiece (modal);

M00 – Program stop;

M02 – End of program;

M03 – spindle on (CW);

M04 – spindle on (CCW).

NB Many more codes/auxiliary functions exist, utilised in

‘word-address format’ programs.*Non–modal commands are

only active in that actual block. [Source, Smith et al., 1993]

Machining and Monitoring Strategies 451

Figure 223. Basic kinematics of the non-orthogonal machine tool, with a simulated rendition of the es-

sential elements of the machine’s structure. [Courtesy of Giddings & Lewis]

squareness errors to be considered, if the machine tool

is to perform within its intended specication. In ef-

fect, with an orthogonal machine tool three axes are

controlled (i.e. the X-, Y- and Z-axes), and the rota-

tional and squareness parameters must be ‘perfect’.

us, these errors/uncertainties can be established by

laser-calibration, then compensated for up to a certain

extent to create and ‘stable machine’. If for example,

one considers the worst case scenario for a machine

tool conguration, namely that of a horizontal ma-

chining centre. Here, if we just deal with one axis,

then the spindle travels up and down the column’s

452 Chapter 9